What is the best software to build a large community platform with a marketplace?

If you want to build a community around a e-commerce model then, I suggest Shopify to attract your community from Facebook otherwise try Magento Connect.

In my personal opinion you should start with a Facebook Group and a Meetup that attracts customers using a blogger outreach program. 

is a breakthrough eCommerce platform allowing business owners to create an online marketplace just from scratch and manage it with maximum efficiency. It supports a wide range of online businesses – from start-ups and small web shops to large shopping malls and even multi-store marketplaces. Currently, over35,000 of established, rapidly growing online ventures are running successfully on the CS-Cart platforms.

Multi-functional, flexible and easy-to-use, our online marketplace software offers all ecommerce essentials as well as plenty of extra tools to provide both vendors and their shoppers with a new, more engaging and rewarding way to make purchases and interact online. It is based on the industry-leading technologies, including PHP scripting, CSS layouts, Smarty template engine, MySQL databases, AJAX-based web interface and many others.